Radio front-end circuit

ABSTRACT

In a radio front-end circuit having at least two amplifier-mixer cascades, the amplifiers are switched on or off via the mixers. By switching between the amplifier-mixer cascades, the power consumption is lowered.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application is a continuation of Application No. 09/004,766,filed on Jan. 9, 1998, the entire disclosure of which is herebyincorporated by reference.

TECHNICAL FIELD

[0002] The invention relates to a radio front-end circuit and morespecifically to a reconfigurable radio front-end circuit.

BACKGROUND

[0003] Radio front-end circuits are typically designed for low-noiseoperation. Such operation requires thetransconductance-source-resistance product, g_(m) ^(.)R_(S), of theinput device, e.g. a bipolar or MOS transistor, of a front-end circuitto be much larger than 1. In its turn, this leads to a high currentconsumption as the quiescent current determines the transconductance ofthe input device.

[0004] Frequently, the signal strength is much higher than thesensitivity level of the front-end circuit. In such cases, the bias ofthe front-end circuit could be reduced in order to lower the supplycurrent. The noise level would increase, but the signal-to-noise ratiocould nevertheless be kept high enough not to degrade the receivedsignal.

[0005] However, changing the front-end circuit bias degrades inputmatching and reduces linearity and band width to such an extent that itis hardly ever used. Lowering the quiescent current of the input devicewill lower the transit frequency of the device. For high-frequencyapplications, such as cellular telephony, this is no option because thetransit frequency margin is too low. The current density of the inputdevice has to be maintained which implies that it will be necessary toswitch between two different front-end circuit structures. The switchnecessary for altering the front-end circuit structure will contributeto the noise level and will typically degrade the low-noise performance.Furthermore, linearity is often also an issue.

SUMMARY

[0006] The object of the invention is to bring about a reconfigurableradio front-end circuit in which the power consumption is reduced whenthe signal strength is high.

[0007] This is attained by means of the front-end circuit according tothe invention in that the amplifiers of the front-end circuit areswitched on or off via the mixers instead of being switched on or off byswitching the bias current or bias voltage of the amplifiers or byintroducing separate switches or multipliers in the signal path.

[0008] The advantage of this is that the input impedance of theamplifier changes less than if its supply current is switched. Moreover,no new circuit elements are introduced in the signal path, which wouldotherwise destroy the noise performance.

BRIEF DESCRIPTION OF THE DRAWING

[0009] The invention will be described more in detail below withreference to the appended drawing, on which the single Figureillustrates one embodiment of a reconfigurable radio front-end circuitin accordance with the invention.

DETAILED DESCRIPTION

[0010] The embodiment of a reconfigurable radio front-end circuit inaccordance with the invention illustrated on the drawing comprises a lownoise amplifier 1 and a low current linear amplifier 2. It is to beunderstood that other types of amplifiers may be used and also that morethan two amplifiers may be used.

[0011] The input terminal of the low noise amplifier 1 and the inputterminal of the low current linear amplifier 2 are both connected to anantenna terminal 3 of the radio which is not shown in any furtherdetail.

[0012] The output terminal of the low noise amplifier 1 is connected toa first input terminal of a low noise mixer 4. A second input terminalof the low noise mixer 4 is connected to a first output terminal of alocal oscillator drive switch 6. The switch 6 has a local oscillatorinput terminal connected to a local oscillator 7, and a control inputterminal connected to a control circuit 8.

[0013] The output terminal of the low current linear amplifier 2 isconnected to a first input of a linear mixer 5. A second input terminalof the linear mixer 5 is connected to a second output terminal of thelocal oscillator drive switch 6.

[0014] It should be pointed out that if more than two amplifiers areused as indicated above, each amplifier is associated with its ownmixer. Moreover, each mixer would be connected to its own outputterminal of the switch 6.

[0015] The control circuit 8 is adapted to control the local oscillatordrive switch 6 to connect/disconnect the mixers 4 and 5 to/from thelocal oscillator 7 in response to the strength of signals received bythe antenna 3 and/or in response to any other parameter, e.g.interference strength.

[0016] In the embodiment shown, the output terminal of the low noisemixer 4 and the output terminal of the linear mixer 5 are connected tothe input terminal of a common intermediate frequency amplifier 9 via alow noise intermediate frequency amplifier 10 and a low currentintermediate frequency amplifier 11, respectively.

[0017] The control circuit 8 in the embodiment shown is adapted tocontrol the local oscillator drive switch 6 in such a manner that thelow noise mixer 4 is disconnected from the local oscillator 7 inresponse to high signal strength and/or high interference strength, andconnected to the local oscillator 7 in response to low signal strengthand/or low interference strength.

[0018] Also, in the embodiment shown, the mixer 4, when disconnectedfrom the local oscillator 7 by means of the switch 6, is adapted tocause low noise amplifier 1 not to consume any power. Alternatively, thecontrol circuit 8 in the embodiment shown can be adapted to switch offthe quiescent current of the low noise amplifier 1 at the same time asthe low noise mixer 4 is disconnected from the local oscillator 7.

[0019] As indicated above, more than two different amplifiers andassociated mixers may be used. In accordance with the invention, itwill, thus, be possible to switch e.g. between two or more low noiseamplifier-mixer cascades depending on signal strength or anotherparameter such as interference strength an mentioned above. As describedabove, one amplifier-mixer cascade could be optimized for low noise,resulting in high power consumption, while the other could be optimizedfor low current and possibly high linearity. It would then be possibleto lower the average power consumption by switching between these twocascades.

[0020] As indicated above, the advantage of this is that the inputimpedance of the amplifier changes less than if its supply current ischanged, and that no new circuit elements are introduced in the signalpath which would otherwise destroy the noise performance.

What is claimed is:
 1. A radio front-end circuit comprising at least twoamplifiers, an input terminal of each amplifier to be connected to anantenna terminal of the radio, an output terminal of each amplifierbeing connected to a first input terminal of an associated mixer, asecond input terminal of each mixer being connected to a localoscillator, and an output terminal of each mixer to be connected to aninput terminal of an intermediate frequency amplifier, and a switchinterconnected between the local oscillator and the respective secondinput terminal of each mixer to connect and disconnect, respectively,the local oscillator to and from, respectively, the respective mixer. 2.The circuit according to claim 1, wherein a control circuit is adaptedto control the switch.
 3. The circuit according to claim 2, wherein thecontrol circuit is adapted to control the switch in response to at leastone of signal strength and interference strength.
 4. The circuitaccording to claim 3, wherein the control circuit is adapted to controlthe switch to disconnect the local oscillator from one of the mixers inresponse to at least one of high signal strength and high interferencestrength, and to connect the local oscillator to that mixer in responseto at least one of low signal strength and low interference strength. 5.The circuit according to claim 4, wherein the switch is adapted toswitch off a quiescent current of the associated amplifier atsubstantially the same time as the local oscillator is disconnected fromsaid mixer.
 6. The circuit according to claim 1, comprising twoamplifiers, wherein one of the amplifiers is a low noise amplifier andthe other is a low current linear amplifier.